1. Field of the Invention
The present invention relates to a data recording method applied to an optical disc that uses EFM modulation.
2. Description of the Related Art
Since optical discs such as a compact disc digital audio (CD-DA) disc and a compact disc read only memory (CD-ROM) are easy to handle and produced at relatively low cost, they have been widely used as recording mediums that store data. In recent years, a compact disc recordable (CD-R) disc and a compact disc rewritable (CD-RW) disc have come out. Data can be easily recorded to such discs. Thus, optical discs in accordance with the CD standard such as a CD-DA disc, a CD-ROM disc, a CD-R disc, and a CD-RW disc are becoming the main stream of data recording mediums. In addition, in recent years, using MPEG1 Audio Layer-3 (MP3) and adaptive transform acoustic coding (ATRAC), audio data is compressed and recorded on a CD-ROM disc, a CD-R disc, a CD-RW disc, or the like.
However, as the CD-R disc and CD-RW disc have come out, data recorded on a CD can be easily copied. Thus, a problem about copyright protection arises. When content data of a CD is recoded, countermeasures are required to protect content data.
FIG. 1 shows an outline of a flow of a copy operation. A reproducing apparatus represented by reference numeral 21 reproduces content data from an original disc for example a CD 22. Reference numeral 23 represents an optical pickup. Reference numeral 24 represents a reproduction signal process portion. Reproduction data is supplied from the reproducing apparatus 21 to a recording process portion 32 of a recording apparatus 31. An optical pickup 33 records the reproduction data on an optical disc for example a CD-R 34. Recorded contents of the original CD 22 are copied to the CD-R 34. Thus, using the reproducing apparatus 21 and the recording apparatus 31, a copy disc of the original CD 22 can be easily produced.
In a CD, to decrease a DC component of a digital signal that is recorded, eight-to-fourteen modulation (EFM) is used. In the EFM, each data symbol of eight data bits is converted into a code symbol of 14 channel bits and merging bits of three bits are added at a boundary of two sequences of 14 channel bits.
A method for prohibiting digital information recorded on a disc from being copied in accordance with a characteristic of a digital modulation system such as EFM is disclosed in the following patent related art reference 1. In the patent related art reference 1, a special encoder and a standard encoder are used. In the standard encoder, digital sum variation (DSV) values cumulate in one direction. In contrast, in the special encoder, DSV values are prevented from cumulating. When the standard encoder re-encodes a predetermined data sequence, DSV values diverge. As a result, digital information cannot be normally reproduced. Thus, digital information can be prevented from being copied.
Patent Related Art Reference 1
Japanese Patent Laid-Open Publication No. HEI 9-288864
As described above, with a particular data sequence (referred to as DSV control data), DSV values are deviated in the plus direction or the minus direction. As a result, it becomes difficult or impossible to read contents from the disc. Consequently, copy detection and copy protection can be performed. However, actually, due to several causes, the expected objective for copy detection and copy protection cannot be accomplished.
As a first problem, in a synchronous pattern that represents the start position of an EFM frame and a sub code region for a sub code, the DSV control data cannot be placed in accordance with the format of a recording medium such as a CD. Even if DSV values are deviated in the plus direction or the minus direction, after data in which a DSV control data sequence cannot be placed, the logic of the EFM encoder causes the DSV values to be kept around zero. As a result, after such data, the DSV values are inversely deviated. As a result, the DSV values zigzag vary in a saw tooth wave shape. Thus, since the read drive or the like recognizes the average value of which the DSV values zigzag vary as DSV values, it treats the average value of the DSV values as 0. Consequently, the DSV values do not sufficiently deviate so that read disability takes place. As a result, the desired objective of which read disability takes place due to deviation of DSV values cannot be accomplished.
As a second problem, depending on the internal controlling method of a CD-R drive that produces a copy disc using a drive for a CD-R or the like, the DSV values may not be diverged. This is because the DSV controlling method of an IC that performs the encoding process such as EFM depends on the drive. Mainly, such a situation of which DSV values do not diverge takes place depending on the DSV counter length and the operation in the case that a DSV value of the disc exceeds the upper limit of the DSV counter.
When a DSV value of data reproduced from a disc reaches the upper limit of the DSV counter, mainly two types of operations are performed. In the first type, a limiter is added to the DSV counter. With the limiter, an upper limit value of a plus value or a minus value is held (hereinafter this type is referred to as limiter type). In the second type, when the value of the DSV counter exceeds the upper limit, the counter value becomes a minus value (hereinafter this type is referred to as overflow type). When the memory that stores a numeric value of an arithmetic operation of a computer has an upper limit, the most significant bit is generally used as a symbol of a minus value.
For example, an eight-bit counter can count up values from (0x00) (where 0x represents hexadecimal notation) to (0xff). The counter uses a code of 2's complement to express plus and minus values. In other words, values from (0x00) to (0x7f) are treated as plus values. Values from (0xff) to (0x80) are treated as minus values. (0xff) represents −1. Thus, (0x80) of which 1 is added to (0x7f) (+127 in decimal notation) that is the upper limit value is a minus value −128, not +128.
In the limiter type, since DSV values are simply deviated in the plus direction or minus direction, if a data sequence in which DSV values cannot be controlled due to a restriction on the format is followed or preceded by a special data sequence, the DSV values can be deviated. However, in the overflow type, even if a special data sequence is placed, when a DSV value of the encoder overflows, the sign of the DSV value is inverted. As a result, DSV values vary zigzag in a saw tooth wave shape. Thus, since the read drive recognizes the average value of DSV values as DSV values, it treats the DSV values as 0. Consequently, since DSV values do not sufficiently deviate or diverge, read disability does not take place. As a result, copy detection and copy protection cannot be sufficiently performed due to deviation of DSV values.
In addition, as described above, if DSV values are deviated in the plus direction or minus direction, it becomes difficult or impossible for the drive to read data from a disc so as to perform copy detection or copy protection. In this case, even in an original disc, DSV values inevitably deviate. As a result, an original disc from which contents can be prevented from being copied cannot be produced.